1. Field of the Invention
The present invention relates to Frequency Modulated Continuous Wave (FMCW) radar and in particular to a coherent FMCW radar device and a method of FMCW radar operation.
2. Discussion of Prior Art
FMCW radar systems are well known and have been widely used in a variety of applications for many years. In such systems the range to a target is measured by systematically varying the frequency of a transmitted radio frequency (RF) signal. Typically, the radar is arranged so that the transmitted frequency varies linearly with time; for example a triangular or saw-tooth frequency sweep is implemented. This frequency sweep effectively places a “time stamp” on the transmitted signal at every instant and the frequency difference between the transmitted signal and the signal returned from a target (i.e. the reflected or received signal) can be used to provide a measure of target range.
It is also well known to those skilled in the art that the accuracy of range measurements made using an FMCW radar depend on the linearity of the frequency sweep. In a typical FMCW radar, a voltage controlled oscillator (VCO) is used to convert a voltage variation into a corresponding frequency variation. Although it is trivial to produce a high quality linear voltage variation (e.g. a triangular or saw-tooth waveform), conversion to the corresponding frequency variation by the VCO often results in the introduction of significant non-linearities that seriously degrade the range resolution of the FMCW radar. A number of different approaches have thus been adopted over the years with the aim of improving the frequency sweep linearity of FMCW radar systems.
For example, attempts have been made to produce VCOs that are inherently linear. In particular, YIG oscillators are now produced by Micro Lambda Wireless Inc, Freemont, Calif., USA in which a fine-tuning coil produces a frequency sweep linearity of, at best, 0.1%. It is also known to modify or pre-distort the voltage tuning signal applied to a standard VCO to compensate for any non-linearity in the VCO response characteristics. A number of analogue and digital pre-distortion techniques have been proposed which can improve VCO linearity to better than about 1%. This level of linearity has been found to be sufficient for certain low cost FMCW radar applications.
It has also been described previously in GB2083966 and GB1589047 how non-linear frequency sweep effects can be reduced by sampling a return signal in a non-linear manner. In particular, GB2083966 and GB1589047 describe how an artificial, fixed range, target may be used to generate a “beat” frequency from which a stream of sampling pulses can be derived. The interval between such sampling pulses would be constant for a perfectly linear frequency sweep, but will vary if the frequency sweep is non-linear. Use of a sample-and-hold circuit to sample the returned signal (i.e. the signal returned by a real target) thus compensates for any non-linearities in the frequency sweep of the transmitted signal.
To date, the most commonly used technique for improving the linearity of FMCW radars is closed loop feedback. Closed loop feedback techniques have been implemented in a variety of ways but they are all based upon creating an artificial target which generates a “beat” frequency when mixed with a reference signal. In a perfectly linearised FMCW radar a fixed range target would produce a constant “beat” frequency. Therefore, in a practical FMCW radar, if the “beat” frequency drifts from the desired constant frequency value an error signal can be generated to fine tune the VCO to maintain a constant “beat” frequency. This feedback technique can be implemented at the final RF frequency of the radar or at a lower, downconverted, frequency. Waveforms having a linearity better than 0.05% have been demonstrated with a bandwidth of around 600 MHz. An example of a closed loop feedback loop arrangement is described in the paper “Novel 24 GHz FMCW Front End with 2.45 GHz SAW Reference Path for High-Precision Distance Measurements” by M Nalezinski, M Vossiek, P Heide, (Siemens AG, Munich), IEEE MTT-S International Microwave Symposium, Prague, June 1997.
It should be noted that FMCW radar systems of the type described above operate incoherently. In other words, the radars only output information related to the frequency shift between the transmitted and returned signals; the radars are not able to measure the difference in phase between transmitted and returned signals. Although an incoherent radar system is suitable for many applications, the provision of a coherent system has many advantages. For example, coherent systems allow Doppler processing to determine information on the velocity of detected targets. Furthermore, coherent integration over N frequency sweeps improves the signal to noise ratio (SNR) by a factor of N. This should be contrasted to the SNR increase of √{square root over (N)} typically obtained using incoherent integration of N frequency sweeps.
Despite the clear benefits of coherent operation, the requirement to not only control the linearity of the frequency sweep but to also control the absolute frequency of each sweep has limited the development of coherent FMCW radar systems. At operating frequencies less than around 20 GHz, VCOs have been produced that have sufficient frequency stability to enable coherent operation. Attempts have also been made to implement coherent FMCW radar using direct digital synthesis (DDS). Although DDS allows the production of repeatable waveforms, it can only provide such waveforms at frequencies up to around 1 GHz. The DDS waveform must then be up-converted or multiplied to the final RF frequency or be included in a phase locked loop circuit. The up-conversion technique involves a local oscillator which will drift with frequency and does not lend itself to coherent operation. The phase locked loop technique can provide coherent operation at frequencies below 20 GHz but the phase noise of the transmitted signal is very poor resulting in poor radar sensitivity. To date, the benefits of coherent FMCW radar operation have thus been restricted to DDS radars operating at frequencies less than 20 GHz.